1. Field of the Invention
The present invention relates to an optical scanning apparatus provided with a light blocking member that blocks other light without blocking the light in an effective scanning range of a laser beam, and also relates to an electrophotographic image forming apparatus such as a copying machine, a printer, or a facsimile machine provided with that optical scanning apparatus.
2. Related Art
In an electrophotographic image forming apparatus of this type, while scanning the surface of a photosensitive body with a laser beam, the intensity of the laser beam is controlled to write a latent image of an image, text, or the like to the surface of the photosensitive body line by line (main scanning line), the latent image on the surface of the photosensitive body is developed using toner to form a toner image on the surface of the photosensitive body, and the toner image on the surface of the photosensitive body is transferred to a recording paper.
An optical scanning apparatus is used for projection of a laser beam. This optical scanning apparatus is provided with a semiconductor laser that emits a laser beam, a rotating polygon mirror that reflects the laser beam from the semiconductor laser to deflect/scan that light, and a plurality of lenses that refract the laser beam that is deflected/scanned by the rotating polygon mirror. The optical scanning apparatus converges and projects the laser beam that is deflected/scanned onto the surface of the photosensitive body via the lenses. While the laser beam is repeatedly scanned in a main scanning direction on the surface of the photosensitive body, the surface of the photosensitive body is moved in a sub scanning direction, so that a latent image is formed on the surface of the photosensitive body.
Ordinarily, a plurality of lenses are provided in the light path of the laser beam from the semiconductor laser to the surface of the photosensitive body, and among the lenses is included an fθ lens, a toroidal lens, or the like. An fθ lens corrects a laser beam that is reflected by the rotating polygon mirror and moves at an equiangular velocity so as to move at a constant velocity on the surface of the photosensitive body. A toroidal lens corrects the light beam of the laser beam to the sub scanning direction.
In an optical scanning apparatus having such a configuration, the laser beam that has been reflected by one reflective face (mirror face) of the rotating polygon mirror is converged and projected onto the surface of the photosensitive body via the plurality of lenses, but at this time, there is the problem that some of the light reflected at the lens surface strays and is again incident on the rotating polygon mirror.
Incidentally, a rotating polygon mirror having six faces is often used in conventional optical scanning apparatuses. In a rotating polygon mirror having six faces, a large angle of 60 degrees is formed by a reflective face that is adjacent to one reflective face in the tangential direction, so even if the laser beam that has been reflected at one reflective face of the rotating polygon mirror is reflected at a lens surface, and strays and is incident on an adjacent reflective face, the stray light reflected at this reflective face proceeds in a direction outside of the effective scanning range of the scanned laser beam (i.e., a direction other than on the surface of the photosensitive body), so there is little need to worry about stray light.
However, recently, rotating polygon mirrors having eight faces have come into use in order to increase the number of revolutions in which it is possible to scan with the same rotational speed, thereby accelerating formation of the latent image on the surface of the photosensitive body. In this case, with a rotating polygon mirror having eight faces, an angle of 45 degrees is formed by a reflective face that is adjacent to one reflective face in the tangential direction, which is 15 degrees smaller than in the case of a rotating polygon mirror having six faces, so when a laser beam that has been reflected at one reflective face of the rotating polygon mirror is reflected at a lens surface, and strays and is incident on an adjacent reflective face, the stray light reflected at this reflective face proceeds in a direction within the effective scanning range of the scanned laser beam (i.e., in the direction of the surface of the photosensitive body), so there is the problem that the stray light affects formation (exposure) of an electrostatic latent image.
Here, problems related to stray light due to using a rotating polygon mirror having eight faces will be described in detail with reference to FIGS. 11 and 12.
FIG. 11 shows how this sort of stray light is incident on a rotating polygon mirror having eight faces. FIG. 12 shows, over time, how one laser beam fired from one light source is reflected by one reflective face of a rotating polygon mirror having eight faces and is deflected/scanned. In FIG. 12, in order to simplify the description, by way of example, three scanning laser beams L2a, L2b, and L2c that move over time are shown.
That is, an incident laser beam L1 from a light source incident on one reflective face (mirror face) S1 of a rotating polygon mirror 74 is reflected by that reflective face S1 to become a scanning laser beam L2 (L2a, L2b, L2c), and is converged (indicated by time-series convergence points a, b, and c) and projected onto the surface (image face) 301 of a photosensitive drum 3 via a first fθ lens 75 and a second fθ lens 77. On the other hand, part of a laser beam reflected by the surface of the second fθ lens 77 becomes stray light L3 (L3a, L3b, L3c), is incident (indicated by time-series incidence points a, b, and c) on a reflective face (mirror face) S2 adjacent to the reflective face (mirror face) S1 of the rotating polygon mirror 74, is again reflected by that reflective face S2 to become a reflected laser beam L4 (L4a, L4b, L4c, . . . ), and is converged (indicated by time-series convergence points a′, b′, and c′) as unnecessary stray light on the surface 301 of the photosensitive drum 3 (referred to below as a photosensitive body surface) by again passing through the first fθ lens 75 and the second fθ lens 77. That is, the one incident laser beam L1 also scans other scanning positions (other convergence points a′, b′, and c′ on the photosensitive body surface 301 shown in FIG. 12) different from the proper scanning positions (convergence points a, b, and c on the photosensitive body surface 301 shown in FIG. 12). The scanning positions on the photosensitive body surface 301 due to this stray light are about 45 to 50 mm in terms of image height. Incidentally, the image height (i.e., the width of an effective scanning region W0 on the photosensitive body surface 301) of the entire photosensitive drum 3 is about 220 mm.
Here, because the transmittance of each lens is about 90 to 95%, it is possible that up to about 10% of the scanning laser beam L2 (L2a, L2b, and L2c) will be incident as stray light on the reflective face S2 of the rotating polygon mirror 74. In this case, although the amount of stray light itself is small, when that stray light is converged within the effective scanning region W0 of the photosensitive body surface 301, formation of an electrostatic latent image is significantly affected. Therefore, it is necessary to block stray light well enough that formation of an electrostatic latent image is not affected. Consequently, there have been proposals in the conventional technology for an optical scanning apparatus in which a light blocking member for blocking stray light near a rotating polygon mirror is disposed (e.g., see JP S62-269925A (referred to below as ‘Patent Document 1’)).
In the optical scanning apparatus described in Patent Document 1, a light blocking member is vertically disposed between a rotating polygon mirror and a first fθ lens, near the rotating polygon mirror. This light blocking member is formed in an arc shape when viewed from above, with the rotational center of the rotating polygon mirror as its center, and both end portions of the light blocking member are disposed close to corner portions at both ends of one reflective face of the rotating polygon mirror. Also, although not described in Patent Document 1, this light blocking member is disposed standing perpendicular to a support member that rotatably supports the rotating polygon mirror.
In this case, because it is necessary for the light blocking member to block only stray light, and not block the effective scanning range of the laser beam reflected by the rotating polygon mirror, it is necessary for the light blocking member to be disposed as close to the rotating polygon mirror as possible.
FIG. 13 illustrates the optimal disposed position of the light blocking member.
As shown in FIG. 12, the stray lights L3a, L3b, and L3c that are incident on the reflective face (mirror face) S2 of the rotating polygon mirror 74 move in the manner of incidence points a, b, and c with the passage of time. Here, in FIG. 13, a range D indicated by a solid line is the effective scanning range (the range in which the effective scanning region W0 in a main scanning direction X of the photosensitive body surface 301 can be scanned), and light must not be blocked in this effective scanning range D. Accordingly, in order to not block light in the effective scanning range D, and also reliably block stray light, a light blocking member 161 may be disposed at a position separated by W1 in FIG. 13. However, with the light blocking member 161 disposed at this position, when the rotating polygon mirror 74 rotates to the position indicated by the double-dotted chained line in FIG. 13, there is a possibility that the corner (portion bordering the adjacent reflective face) of the rotating polygon mirror 74 will make contact with the light blocking member 161, so it difficult in practice to dispose the light blocking member 161 at the position separated by W1.
On the other hand, it is not absolutely necessary to block 100% of stray light; there is no problem if some amount of stray light reaches the photosensitive body surface 301 as long as that amount of stray light does not affect formation of an electrostatic latent image. More specifically, if about 50 to 80% of the stray light can be blocked, formation of an electrostatic latent image will not be affected. Consequently, in order to not block light in the effective scanning range D, and also reliably block stray light such that the stray light does not affect formation of an electrostatic latent image, the light blocking member 161 may be disposed at a position separated by W2 in FIG. 13. With the light blocking member 161 disposed at this position, it is possible to block stray light such that the stray light does not affect formation of an electrostatic latent image, and there is no worry that a corner of the rotating polygon mirror 74 will make contact with the light blocking member 161 when turning. Note that, in consideration of preventing contact of the rotating polygon mirror 74 and the light blocking member 161, it is also conceivable to dispose the light blocking member 161 at a position separated by W3 in FIG. 13, but when the light blocking member 161 is disposed at this position, the percentage of stray light that is blocked decreases (e.g., decreases to about 30%), and so the amount of stray light that reaches the photosensitive body surface 301 without being blocked increases. Therefore, there is a possibility that formation of an electrostatic latent image will be affected. Incidentally, the distance W2 in FIG. 13 is about 3 to 5 mm.
On the other hand, there is the problem that when the light blocking member is disposed near a reflective face of the rotating polygon mirror (disposed at the position separated by W2 in FIG. 13), because the rotating polygon mirror rotates at high speed (specifically, about 40,000 rpm), the light blocking member 161 disposed near the rotating polygon mirror 74 vibrates, producing high-pitched noise.
That is, in the rotating polygon mirror 74, as shown in FIG. 14(a), distances T from a rotational center O to a reflective face S of the rotating polygon mirror 74 differ between the parts of the reflective face S. A distance T1 from the rotational center O to each corner of the reflective face S, which are portions that border an adjacent reflective face S, is the longest distance, and a distance T2 from the rotational center O to the center point of the reflective face S is the shortest distance.
When the rotating polygon mirror 74 having such a shape rotates at high speed, as shown in FIG. 14(a), this rotation of the rotating polygon mirror 74 is accompanied by rotation of the surrounding air in a donut shape so as to draw a circle in the same direction (in the drawings, arrow R indicates the flow of air). This flow of air (airflow) R, when viewed in the vertical direction, as shown in FIG. 14(b), flows downward, and flows along the surface of a support member 60 of the rotating polygon mirror 74 and expands into the surrounding area.
When, as in above Patent Document 1, the light blocking member 161 is provided standing near the rotating polygon mirror 74, which causes this sort of airflow, a shortest distance T11 between the light blocking member 161 and the rotating polygon mirror 74 occurs when a corner of a reflective face S of the rotating polygon mirror 74 is facing an opposing face 161a of the light blocking member 161, as shown in FIG. 15(a), and a longest distance T12 between the light blocking member 161 and the rotating polygon mirror 74 occurs when a reflective face S of the rotating polygon mirror 74 is facing the opposing face 161a of the light blocking member 161 so as to be parallel, as shown in FIG. 15(b). Accordingly, when the rotating polygon mirror 74 rotates at high speed, periodically with that high speed rotation, the distance between the opposing face 161a of the light blocking member 161 and a reflective face S of the rotating polygon mirror 74 repeats between near (distance T11) and far (distance T12). Therefore, the air (airflow) R that flows between the opposing face 161a of the light blocking member 161 and a reflective face S of the rotating polygon mirror 74 repeatedly is in a sparse/dense state depending on whether a reflective face S of the rotating polygon mirror 74 is near or far from the light blocking member 161. Also, as shown in FIG. 15(c), when the light blocking member 161 is provided standing vertically on the support member 60, air that attempts to flow to the outside and downward is directly stopped by the vertical face of the light blocking member 161, so the flow of air is completely prevented, and a large air pressure is applied to the light blocking member 161. Due to such a large air pressure, and the repeating sparse/dense state of the airflow described above, the light blocking member 161 vibrates, causing a high-pitched vibration noise.
Vibration noise that is comparatively low-pitched, such as motor revolution noise, does not sound particularly harsh to human ears, but high-pitched vibration noise sounds very harsh to human ears. Therefore, when using an optical scanning apparatus in which a light blocking member is disposed near a rotating polygon mirror, as with the conventional technology, there is the problem that high-pitched noise causes discomfort to a user.